HUDs in vehicles usually consist of an imager and a mirror system with two or more mirrors, which are located together in the dashboard of the vehicle. In an HUD, the image information radiated by an imager is made accessible to a viewer, for example the driver of a motor vehicle or the pilot of an aircraft, as a virtual image in his viewing area. The size of the image generated by the HUD within the viewing area of the viewer is usually referred to as a field of view (FOV). The eyebox is called the area which defines the image-side opening angle and at which the different main and edge beams of the different field points intersect. Usually, the visibility of the displayed image information is limited to a certain spatial area within the vehicle so that the viewer should be at least with one eye within the head motion box (HMB) defined by the imaging system used. The HMB is usually rectangular with a size of approximately 220×80 mm2. For current HUDs, the HMB is identical to the eyebox. The desired size of the eyebox and the FOV results in the necessary etendue, which must be provided by the imager.
The image sensor is used for the display and radiation of image information. A corresponding individual component is designated as the imager. This can be, for example, a TFT/LCD display or a pico projector. The imager usually comprises a so-called imager (image generator) for displaying the image information. Mostly TFT/LCD panels, DMD, LCoS, MEMS or similar are used as possible imagers. The imager can preferably be a pico projector with DMD/LCoS as an imager or also a simple TFT panel. If a TFT panel is used, the image information radiated by the TFT panel is made accessible to the viewer directly as a virtual image in the field of view. If a pico projector is used, it first generates an intermediate image on an additional intermediate screen, whereby this intermediate image essentially takes the function of a TFT panel. The intermediate image is then usually imaged into the eyebox of the viewer by a mirror system.
Incident radiation from outside the HUD into an HUD, in particular solar radiation, can lead to reflections at the imager or at other parts of the HUD imaging system. These reflections are particularly dangerous in vehicles, since this can lead to a massive impairment of the driver by glare and thus to the loss of control over the vehicle. The remaining occupants of a vehicle could also be blinded by the reflections, which have a negative effect on the general acceptance of HUDs in such vehicles. In addition, direct sunlight can lead to a strong heating up to an overheating, in particular, of TFT panels of actual HUDs, which negatively affects the expected lifetime of the imager. However, thermal influence also leads to the risk of overheating in HUD systems with diffusers, which can lead to changes in the material of the diffuser.
In order to increase the lifetime of the TFT panels used in HUDs, the TFT panel must therefore be protected from excessive sunlight. This can be done, in particular, if strong sunlight occurs, the TFT panel is dimmed or completely switched off, a cover is placed over the TFT panel or the HUD, or an optical component of the HUD is tilted for radiation deflection so that the incident solar radiation can be deflected in a light trap. For this purpose, for example, a critical temperature of at least one component of the HUD can be monitored. Furthermore, the intensity of an incident solar radiation on the TFT panel can be measured or estimated.
WO 2015/162836 A1 discloses an HUD having an estimator which estimates the relative position of the sun relative to its geographical position. The intensity of incident radiation on a TFT panel can then be estimated from a sun position calculated therefrom. Depending on the estimated radiation intensity, a control of the incident solar radiation on the TFT panel and an adjustment of the brightness of the TFT panel can then take place.
US 2015/0323793 A1 discloses an HUD with an automatic switch-off device, in which, when the HUD is not used, one of the mirrors of the HUD is placed in a protected position. In this way, the imager of the HUD can be protected against damage.
US 2015/0098029 A1 discloses an HUD in which a direct measurement of the intensity of incident solar radiation is carried out with the aid of an infrared radiation detector. For this purpose, one of the mirrors of the HUD is partially transparent to the infrared portion of a incident radiation on the mirror. In particular, the mirror can be a mirror which is reflecting in the visible spectral range and is at least partially transparent in the infrared. Depending on the measured intensity of an incident radiation, the background brightness of a TFT panel can be adjusted. Due to the special coating of the mirror, however, this method usually results in significantly higher production costs compared to conventional mirrors.
In order to avoid a glare effect by such reflections in current HUDs, in particular, the imager or an existing diffuser is not arranged at right angles to the main beam. Instead, they are positioned tilted relative to the main beam in the beam path of the HUD, because otherwise an incident solar radiation perpendicular to these components could blind a viewer or other persons. By means of a tilted arrangement, occurring reflections can be deflected into a light trap without glare. Furthermore, it is excluded that sunlight can fall on the imager at such an angle that it is reflected to the viewer.
If the imager is not adapted to such applications, less light would be available for the image than in the right-angled case. However, the light output can be improved by either obliquely illuminating the imager or adding an additional component (for example a foil with prisms) to deflect the light. Both variants, however, complicate the imager and thus increase its production costs. If, however, the imager is used telecentrically, which allows a better light output with lower production costs, a reflection of vertically incident solar radiation on the display must be reduced, for example, by deflecting the sunlight into a light trap via one of the mirrors of the HUD. Although in this case the virtual image of the HUD is no longer visible to the driver of a vehicle, but he is not blinded by the sun and can continue to observe the traffic.
However, future HUDs in vehicles will be based on Pico projectors, which no longer generate an intermediate image. The etendue is not enlarged (that means it is not a projector with a diffuser), but only a very small eyebox is illuminated, which must retrace the movements of the vehicle driver. Furthermore, future HUDs will not be installed exclusively in the dashboard of a vehicle, but, for example, partly in an area directly below the vehicle roof. This saves installation space in the dashboard due to the compact design of the entire imaging system. As a result, larger FOVs than in the past can be realized. In such directly projecting HUDs, however, tilting of the imager in order to avoid reflections may not be possible anymore. For example, a DMD as an imager must usually be used telecentrically in order to ensure an optimal and uniform illumination, in particular of the different colors of the DMD. For that matter the main beam of the system strikes the DMD perpendicularly.
Such a protective measure serves both the general traffic safety as well as the personal eye protection of a vehicle driver, whereby at the same time the imager can also be protected against damage caused by solar radiation. In order to avoid unnecessary erroneous triggering of the protective device, however, it is necessary, in addition to the actual intensity of an incident radiation into the HUD, to also recognize its potential glare effect on the driver of a vehicle or the rest of his occupants, and to initiate a corresponding protective measure only if necessary.
It is therefore an object of the present invention to provide a detection device, an HUD and a method for operating an HUD which overcome the described disadvantages of the prior art and which in particular allow, with little additional effort and at a low cost, to angle-selectively and direction-selectively detect an incident radiation in the imaging system of an HUD, whereby the occurrence of a possible glare effect leads to an adaptation of the HUD which at least attenuates the glare effect.